Various types of fuel cell systems that use a fuel cell generating electric power through an electrochemical reaction between a fuel gas and an oxidant gas as an energy source have been developed heretofore. In many of such systems, the fuel cell has a stack structure with numerous cells, acting as minimum units of power generation, stacked therein. Each cell has an MEA (membrane-electrode assembly) in which an air electrode and a fuel electrode are disposed on both sides of an electrolyte membrane formed of an ion exchange membrane, and also has a pair of separators arranged on both sides of the MEA.
In such fuel cell system having a stack-structure fuel cell, it is necessary to constantly monitor each cell to determine whether the cell is exhibiting the desired power-generation performance, and thus, such fuel cell system is usually equipped with a cell monitor. Since the number of stacked cells is large, e.g., several tens or several hundreds, some cell monitors monitor the output voltage in groups, with one group being composed of several cells, instead of detecting the output voltage of every cell.
However, such voltage monitoring in groups cannot identify the minimum cell voltage, even though it can identify a group which includes a cell with a voltage decreasing to a value equal to or lower than a predetermined voltage value. As a solution to the above, for example, the technique disclosed in cited reference 1 estimates the minimum cell voltage at a value obtained by subtracting an average cell voltage from a minimum group voltage.